A. Technical Field
The present invention relates to a process for producing acrylic acid. Specifically, it relates to a process for producing acrylic acid, comprising the step of subjecting acrolein or an acrolein-containing gas to catalytic gas phase oxidation with molecular oxygen or a molecular-oxygen-containing gas using a fixed-bed shell-and-tube reactor as packed with a catalyst.
B. Background Art
As to catalysts which are used in the case of producing acrylic acid by subjecting acrolein or an acrolein-containing gas to catalytic gas phase oxidation, many propositions are made. For examples, JP-B-26287/1969, JP-B-25914/1975, JP-B-54172/1982 and JP-A-218334/1991 are cited. Of these catalysts, some may give acrylic acid with a yield attaining a considerably high level in industrial view, but the below-mentioned problems arise in the case of producing acrylic acid industrially by using these catalysts.
For example, since the industrial production is required to increase the productivity of acrylic acid which is the aimed product, there are generally adopted methods in which the concentration of acrolein which is a raw material is raised or the space velocity is raised. However, the catalytic gas phase oxidation reaction is accompanied by extraordinary heat generation, therefore, under such conditions with a high load, the temperature of a hot spot portion (a locally and extraordinarily high temperature portion in a catalyst layer) goes up by increase of the reacting quantity. As a result, the excess oxidation reaction causes a fall in the yield, or accelerates thermal deterioration of the catalyst, or, in the worst case, may cause a runaway reaction.
Accordingly, to suppress the accumulation of heat in the hot spot portion is very important both for producing acrylic acid in a high yield industrially and for enabling stable operation for a long time by suppressing the deterioration of the catalyst.
As to means to suppress the temperature of the hot spot portion to a low one, several propositions are made. For example, the following methods are proposed: {circle around (1)} a method which involves diluting a catalyst layer, as placed on the gas inlet side, with an inert substance (JP-B-30688/1978); {circle around (2)} a method which involves making the ratio of supporting a catalytically active substance (ratio by weight of the active substance per unit weight of the catalyst) become higher gradually from the gas inlet side toward the gas outlet side (JP-A-10802/1995); {circle around (3)} a method which involves making the size of the catalyst become smaller gradually from the gas inlet side toward the gas outlet side (JP-A-241209/1997); and {circle around (4)} a method which involves lowering the activity of a catalyst, as placed on the gas inlet side, by adding an alkaline metal (JP-A-336060/2000).
However, in all the above-mentioned conventional methods {circle around (1)} to {circle around (3)} to suppress the temperature of the hot spot portion to a low one, since the amount of the catalytically active substance placed on the gas inlet side becomes smaller than that on the gas outlet side, the catalyst placed on the gas inlet side deteriorates faster than that placed on the gas outlet side. As a result, although in view of suppressing the temperature of the hot spot portion to a low one the improvement is achieved in some degree, there arises a problem such that it becomes impossible to continue the reaction for a long time with a high yield kept. Especially, when the reaction is done under conditions with a high load such as using a high concentration of acrolein which is a raw material, this problem becomes striking.
In addition, in the above-mentioned conventional method {circle around (4)}, in view of lowering the activity of the catalyst by “adding” the alkaline metal, the amount of the catalytically active substance placed on the gas inlet side is substantially the same as that on the gas outlet side. However, the addition of the alkaline metal results in decreasing the catalytically active sites, therefore such a catalytic function as corresponds to the amount of the existing catalytically active substance cannot be displayed fully. Accordingly, although the conventional method {circle around (4)} provides more improved results than the conventional methods {circle around (1)} to {circle around (3)}, the conventional method {circle around (4)} has not yet succeeded in fully solve the problem such that it becomes impossible to continue the reaction for a long time with a high yield kept.